This section provides background information related to the present disclosure, which is not necessarily prior art.
Due to the increasingly stringent fuel economy and CO2 emissions regulations, automakers are developing numerous technologies to reduce fuel consumption. For example, exhaust heat recovery systems are used to facilitate engine warmup, which improves fuel economy. Such systems typically include a heat exchanger, which transfers heat from exhaust gas to engine heat transfer fluid during a cold engine start. This shortens the time needed for heat transfer fluid to reach its optimal operating temperature and, by extension, the time required for the engine to warm up and reach its optimal operating temperature.
With exhaust heat recovery systems it is not desirable to have a heat exchanger operational at all times, because the engine may overheat and/or the engine heat transfer fluid may be damaged by high exhaust temperatures. Existing exhaust heat recovery systems typically address these issues by using a diverter valve that opens when the engine is sufficiently warm to direct exhaust to bypass the heat exchanger, which prevents the heat transfer fluid from overheating. This approach places the heat exchanger in parallel with the normal exhaust system routing, which undesirably adds to the packaging space (which can be a bigger problem for vehicles with larger exhaust system components, such as pickup trucks), adds cost due to the need for exhaust diverter valves (which are typically stainless steel in order to provide resistance to temperature and corrosive exhaust gasses), and changes the exhaust back pressure characteristics, which typically complicates the engine calibration process.
The present teachings advantageously provide for an exhaust heat recovery system that does not intrude into the exhaust system itself (or minimizes such intrusion if exhaust side heat transfer enhancements such as multiple flow paths and/or exhaust side fins are included), or require dual exhaust system pathways (one for when exhaust heat recovery is operational, and one for when it is not operational) as found in conventional systems. In contrast to current systems, the present teachings advantageously remove heat transfer fluid from the exhaust heat recovery heat exchanger when heat recovery is not needed, and return the heat transfer fluid into the heat exchanger when exhaust heat recovery is desired, which prevents overheating of the heat transfer fluid. Furthermore, in one embodiment the present teachings advantageously combine exhaust heat recovery and heat storage into a single system, as explained in detail herein. The present teachings are particularly advantageous because exhaust heat recovery and heat storage are the two most effective ways to improve fuel economy by thermal management. One skilled in the art will appreciate that the present teachings provide for numerous additional advantages in addition to those specifically described, as well as numerous unexpected results.